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Do you have brake lights when the trailer is not connected? I know it would be a huge coincidence, but have you checked both bulbs? Prop a stick on your brake pedal and check for voltage at your connectors. Wiggle the bulbs in the connector also. I've had a few instances where my "light out" indicator came on and it was just a loose bulb.

I have checked both rear tail light units, changed all of the bulbs, checked the third light unit on the cab, checked the harness for crimps or binding that might cause a short, pulled and checked every fuse in the IPM. Nothing, then when I hooked up the trailer, pronto...brake lights on the trailer! Very frustrated!!
I have been advised by a friend that there is a brake light module under the hood near the IPM that could be the problem, but I have no idea where it is or what it looks like.

Brake light module? That's new one on my. Try downloading the factory manual that tjbiker03 has provided and see if you can find anything in there about this brake light module. Here is the link to them.

Looking at my wiring diagram, there is a controller called the anti-lock brake controller, but I really don't think that has anything to do with the brake lights. I guess you brake light switch a the pedal could be outta whack, but I think that would effect the trailer lights too. But, I would check it first before moving on to anything else.

Have similar problem. Third brake light works but nothing a rear
Tested with ground, bulb out at back and no power. Cleaned MFS on steering column as well. Power at pedal switch . IPM? any other ideas?